The invention relates to a device and a method for surface treatment, in which the material which is to treated is guided through a bath, where it is treated chemically, by electroplating and/or electro-phoretically.
Methods and devices of this type are used to provide items which are composed at least partially of a conductive material with a surface whose technical or visual properties are adapted to the particular application. This may be carried out by chemical or electrochemical treatment of the surface, for example by chromating the surface of a metallic work piece, and by electrolytic or electrophoretic application of a coating layer.
The known wet methods are extremely complex, since the material which is to be treated has to be guided through a series of vessels which are arranged in series and contain a very wide variety of treatment liquids, for example cleaning agents, electrolytes and electrophoretic enamels. This is carried out discontinuously by immersion of individual items or batches of small parts or in continuous installations using conveyors which move the material which is to be treated individually or, if small parts are involved, in a successive series from immersion basin to immersion basin, through a treatment line. By way of example, a conveyor which holds the material which is to be treated is suitable for this purpose. An appropriate device is known from German Patent 41 42 997.
Since the chemical surface treatment is a time-dependent operation both in the case of electrolytic coating and in the case of electrophoretic coating, a specific residence time of the material which is to be treated is required in the individual immersion basins depending on the particular treatment agent. This residence time, which differs considerably in individual cases, at a given conveying or passage rate, determines the treatment distance required, and consequently the dimensions of the individual immersion basins in the direction of transport.
Since individual large pieces of material which is to be treated and small parts, in order to obtain a homogeneous surface, can only be guided through the individual baths with a limited bed height, conventional installations are extraordinarily expensive and have a comparatively low throughput.
A particular problem consists in achieving a surface treatment which is uniform on all the parts, for example electroplating and/or enameling over the entire surface without flaws. Flaws of this nature are formed in particular at the points of contact between the material to be treated and the conveyor means for transporting the material to be treated through the individual baths. Suitable conveyor means are, in particular for small parts or bulk material, drums although in some cases conveyor belts and, for relatively large parts, special transport cages which are designed according to the configuration of the material to be treated and are therefore expensive, are also suitable. Medium-sized and large parts can also be moved through the individual immersion vessels with the aid of a conveyor belt; in this case, however, the parts have to be arranged one behind the other on the conveyor, in order to make their surfaces as accessible as possible to the treatment agent. However, an arrangement of this nature in a single plane entails the drawback that the throughput is reduced considerably compared to cage transport with parts which are arranged one above the other within certain limits.
The invention is therefore based on the problem of improving the economic viability of the known methods and devices without detriment to the surface quality of the material which is to be treated.
The solution to this object is based on the idea of multiplying the throughput at a given conveying rate, in the individual immersion basins by guiding the material which is to be treated through the individual immersion basins in a plurality of planes one above the other. This can be achieved with the aid of a movable stepped cage, between the steps of which treatment elements, for example stationary electrodes or nozzles for treatment liquid or drying air, which are arranged in the same pattern in a plurality of planes, are arranged.
In this way, the material which is to be treated is guided past the stationary treatment elements with its surface made as accessible as possible, and the throughput is increased as a function of the number of conveying planes or steps.
Moreover, the method according to the invention opens up the possibility of acting on the material to be treated from above and below, for example by spraying or blowing on drying air. Furthermore, an improved layer thickness distribution is achieved, since the electrodes are arranged in a linear manner with respect to the material which is to be treated.
In order to achieve a surface which is as uniform as possible, without flaws, the material which is to be treated can be rearranged within the bath or between two baths, in order to expose points of contact and make them accessible to the treatment agent. The rearrangement may be effected by vibration, with the aid of magnets or with the aid of a rake, the fingers of which engage, in the manner of a comb, between rods or lamellae of a support for the material which is to be treated, for example of a tray.
A device in which the material to be treated is arranged in a lying or suspended position in several planes on or at the stepped cage of a conveyor which guides the material to be treated at least through one vessel, for example an immersion vessel, is particularly suitable for carrying out the method according to the invention. The material which is to be treated is preferably located on a tray and can be deposited in the stepped cage by a supply conveyor, preferably a double-strand conveyor. In order to be able to feed the individual steps or levels of the cage, the stepped cage preferably comprises a vertically displaceable carriage with bearing arms which are arranged at a distance from one another. In this way, the bearing arms can be moved successively into the conveyor plane, in order to lift off and thus pick up a lamellate tray from the conveyor.
For its part, the lifting carriage may be guided in a horizontally displaceable cage trolley which moves the cage through the particular coating vessel.
The fact that the material to be treated, during loading of the stepped cage, moves successively, i.e. at intervals, into the immersion bath does not cause any disadvantage, and in particular does not result in any difference in residence time, if during this phase no current is being applied to the material to be treated.
In this way, the material to be treated can be moved gently into the immersion bath, since it is moving vertically downward and, due to its weight, is held securely on its support.